Method for controlling an electrically heatable catalytic converter and exhaust-gas after-treatment system

ABSTRACT

The invention relates to a method for controlling an electrically heatable catalytic converter for after-treating exhaust gases of an internal combustion engine. The catalytic converter is deactivated or not activated during the start of the engine if the temperature (T KAT (t abst )) of the catalytic converter exceeds a critical temperature T KRIT  when starting the engine. The temperature (T KAT (t abst )) of the catalytic converter is estimated via a temperature model. The invention also relates to an exhaust-gas after-treatment system of an internal combustion engine for carrying out the method of the invention.

FIELD OF THE INVENTION

[0001] The invention relates to a method for controlling an electricallyheatable catalytic converter for after-treating exhaust gases of aninternal combustion engine as well as an exhaust-gas after-treatingsystem for carrying out the method of the invention.

BACKGROUND OF THE INVENTION

[0002] To satisfy strict exhaust-gas requirements, it is necessary thatthe catalytic converter reaches its operating temperature in a shorttime after start (that is, that the light-off temperature threshold isexceeded) and therefore the conversion of the exhaust gases begins. Toaccelerate this warm-up operation, electrically heatable catalyticconverters are used which are hereinafter referred to as E-catalyticconverters. These catalytic converters usually have a heating power >5kW and reach a carrier temperature in the region of 500° C. Fordetermining the temperature of the E-catalytic converter, a temperaturesensor is provided in known E-catalytic converters which supplies theoperating temperature thereof to a control apparatus. Driving theE-catalytic converter then takes place in dependence upon the measuredtemperature of the E-catalytic converter.

SUMMARY OF THE INVENTION

[0003] It is an object of the invention to provide a method via which aknown E-catalytic converter can be simplified with respect to itsconfiguration. Furthermore, the problem of a thermal overload andtherefore damage of the E-catalytic converter is, on the one hand,avoided and, on the other hand, the least possible energy is to be takenfrom an electric energy store for warm-up. It is another object of theinvention to provide an exhaust-gas after-treatment system for carryingout the method of the invention.

[0004] The method of the invention is for controlling an electricallyheatable catalytic converter for after treating exhaust gases of aninternal combustion engine. The method includes the steps of:deactivating or nonactivating the catalytic converter during the startof the engine when the temperature (T_(KAT)(t_(abst))) of the catalyticconverter exceeds a critical temperature T_(KRIT); and, utilizing atemperature model to estimate the temperature (T_(KAT)(t_(abst))) of thecatalytic converter.

[0005] The method according to the invention for controlling (open loopand/or closed loop) an E-catalytic converter affords the advantage thata temperature, which is measured directly on the E-catalytic converter,is not included in the drive of the E-catalytic converter. Rather, thetemperature of the E-catalytic converter is determined by physicalcondition variables without providing a temperature sensor on or in theE-catalytic converter. This notwithstanding, a thermal overload of theE-catalytic converter is avoided in accordance with the invention and/oronly so much energy is drawn from an electric energy store when warmingup the E-catalytic converter as is necessary. The invention thereforeaffords the advantage that by making a temperature sensor in theE-catalytic converter unnecessary, the disadvantages of an E-catalyticconverter control can be ameliorated by including additional physicalcondition variables.

[0006] For the above, three different intervention measures are utilizedin the controlled sequence. The most significant intervention measure isthe deactivation or non-activation of the E-catalytic converter in thecondition wherein it is still hot whereby damage to the E-catalyticconverter is prevented.

[0007] With the possibility of the targeted adaptation of the heatingduration to the existing requirements, electric energy can be saved anda deterioration of the components of the E-catalytic converter can becountered.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will now be described with reference to thedrawings wherein:

[0009]FIG. 1 is a schematic block diagram showing the functionality of afirst intervention possibility;

[0010]FIG. 2 is a block circuit diagram showing a thermal E-catalyticconverter model; and,

[0011]FIG. 3 is a schematic showing an exhaust-gas after-treatmentsystem of an internal combustion engine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0012] The method of the invention provides for different interventionpossibilities on the controlled sequence of the drive of the E-catalyticconverter.

[0013] (a) Condition-Dependent Deactivation of the E-Catalytic Converterduring the Start of the Internal Combustion Engine

[0014] A deactivation of the E-catalytic converter takes place in orderto protect components when the temperature T_(KAT)(t_(abst)) of theE-catalytic converter already exceeds a critical temperature T_(KRIT)during the start. The temperature T_(KAT)(t_(abst)) can be estimated viathe following model the description of which is provided hereinafter.

T _(KAT)(t _(abst))=[T _(KAT)(0)−T _(U)]·exp(−r·c·t _(abst))+T _(U)  (see A8 with qu=0).

[0015] For the estimation, the following must be known: theinstantaneous switchoff duration t_(abst) from the time point of thelast switchoff up to the restart of the engine, the ambient temperatureT_(U) and the temperature T_(KAT)(0) of the E-catalytic converter at thetime point of switchoff.

[0016] The switchoff duration T_(abst) can either be read in via acombination instrument or can be determined, in the manner of asubstitute, from a model of a cool-down characteristic of the engine.

[0017] The ambient temperature T_(U) can be estimated from theintake-air temperature. In the sense of a worst-case estimate, one canalso use a constant substitute value for the ambient temperature T_(U).

[0018] The temperature of the E-catalytic converter T_(KAT)(0) at thetime point of switchoff can be derived from an exhaust-gas temperaturemodel. A typical maximum operating temperature for T_(KAT)(0) can beapplied in the sense of a worst-case estimate.

[0019] In total, the functionality shown in FIG. 1 results for theintervention possibility (a). When the E-catalytic converter temperatureT_(KAT)(t_(abst)) already exceeds a critical temperature T_(KRIT) duringthe start, the E-catalytic converter is not activated. If thetemperature T_(KAT)(t_(abst)) of the E-catalytic converter lies duringthe start below a critical temperature T_(KRIT), then the E-catalyticconverter is activated. The values T_(KAT)(0) and/or T_(U) can also bereplaced by constant values. These values are then dropped as variablequantities.

[0020] (b) Optimization or Shortening of the Heating Duration of theE-Catalytic Converter

[0021] Although the E-catalytic converter is always operated at aconstant heating power, the heat characteristic can run very differentlyin dependence upon the start temperature and the ambient temperature.The heating duration should be adapted to the start individually forreasons of protecting of components and for saving energy.

[0022] If no temperature information is available, then the heatingcharacteristic and therefore the necessary heating duration can bederived from a temperature model. For the temperature trace when heatingup, the following relationship results: $\begin{matrix}{{T_{KAT}(t)} = {{\left\lbrack {{T_{KAT}(0)} - \left( {\frac{q_{zu}}{r} + T_{U}} \right)} \right\rbrack \cdot {\exp \left( {- {rct}} \right)}} + \left( {\frac{q_{zu}}{r} + T_{U}} \right)}} & \text{(A8)}\end{matrix}$

[0023] In addition to the physical condition variables of ambienttemperature T_(U) and the temperature T_(KAT)(0) of the catalyticconverter at start already mentioned in method (a), the heat entryq_(zu) is needed.

[0024] The heat entry q_(zu) comes essentially from the electricalheating power. This can either be measured by measuring the heatingvoltage and the heating current or, by the way of substitution, can alsobe estimated as a constant value.

[0025] The necessary heating duration T_(EKAT) up to reaching thenecessary switchoff temperature T_(KATAB) can be determined in thecontrol apparatus either via explicit solution of the above-mentionedmathematical relationship or via continued computation of T_(KAT) withthe instantaneously passed heating duration as argument t and subsequentcomparison of the result to T_(KATAB).

[0026] The explicit solution yields the following heating duration:$\begin{matrix}{t_{ekat} = {\frac{1}{r \cdot c} \cdot {\ln \left( \frac{{T_{KAT}(0)} - \frac{q_{zu}}{r} - T_{U}}{T_{KATAB} - \frac{q_{zu}}{r} - T_{U}} \right)}}} & \text{(A9)}\end{matrix}$

[0027] (c) Rapid Switchoff of the E-Catalytic Converter for a DefectiveOperation

[0028] An activation of the E-catalytic converter drive is to besuppressed or an existing E-catalytic converter drive is to be endedprematurely under specific conditions for reasons of safety. Thespecific conditions include:

[0029] Diagnosis fault. If the diagnosis of the feed lines to theE-catalytic converter shows a fault, then an activation of theE-catalytic converter is suppressed during start.

[0030] Ambient temperature. At low temperatures, the danger of theE-catalytic converter freezing is present. Under these conditions, anactivation of the E-catalytic converter during start is likewise to besuppressed.

[0031] Engine operation. If the engine stalls during the heating phaseof the E-catalytic converter, then the E-catalytic converter isadvantageously switched off prematurely.

[0032] The model used is described in greater detail in the followingwherein the following overview presents the variables used in thedescription of the model: Abbreviation Explanation Unit T_(KAT) (t)catalytic converter temperature [K] Q_(KAT) (t) thermal heat quantity inthe E-catalytic [J] converter T_(U) (t) ambient temperature [K] q_(zu)(t) inflowing thermal heat flow [J/s] q_(ab) (t) outflowing thermal heatflow [J/s] c thermal proportionality constant [K/J] r thermal transitionresistance [J/K*s] t_(abSt) time elapsed since switchoff of the engine[s]

[0033] Relationship of the catalytic converter temperature and heatquantity:

[0034] The temperature T_(KAT)(t) of the E-catalytic converter is, in afirst approximation, proportional to the inner thermal heat quantityQ_(KAT)(t). The following relationship results when using the specificthermal proportionality factor c:

T _(KAT)(t)=c·Q _(KAT)(t)   (A1)

[0035] Supplied Heat Quantity:

[0036] The heat quantity q_(zu) (t) , which flows to the E-catalyticconverter during the start, originates from the electric heating powerof the E-catalytic converter as well as the heat quantity of theexhaust-gas mass flow. The E-catalytic converter is always driven withthe same power and the heat quantity of the exhaust-gas mass flow isapproximately reproducible after start in idle. For this reason,q_(zu)(t) is viewed as being constant in the following:

q _(zu)(t)=q _(zu)=const   (A2)

[0037] Out-flowing Heat Quantity:

[0038] A portion of the heat quantity, which is stored in theE-catalytic converter, flows off from the E-catalytic converter in theform of a heat flow q_(ab)(t) because of the thermal drop between thetemperature T_(KAT)(t) in the E-catalytic converter and the ambienttemperature T_(U).

q _(ab)(t)=r·(T _(KAT)(t)−T _(U))   (A3)

[0039] Presentation of the Characterizing Differential Equation:

[0040] A change of the inner heat quantity Q_(KAT)(t) in the E-catalyticconverter results exclusively from the in-flow and out-flow of thermalquantities: $\begin{matrix}{\frac{\partial{Q_{KAT}(t)}}{\partial t} = {q_{zu} - {q_{ab}(t)}}} & \text{(A4)}\end{matrix}$

[0041] wherein: q_(ab)(t) from equation A3 is inserted into equation A4:$\begin{matrix}{\frac{\partial{Q_{KAT}(t)}}{\partial t} = {\left( {q_{zu} + {rT}_{U}} \right) - {{rT}_{KAT}(t)}}} & \text{(A5)}\end{matrix}$

[0042] wherein: T_(KAT)(t) from equation A1 is inserted into equationA5: $\begin{matrix}{{\frac{\partial{Q_{KAT}(t)}}{\partial t} + {r \cdot c \cdot {Q_{KAT}(t)}}} = \left( {q_{zu} + {r\quad T_{U}}} \right)} & \text{(A6)}\end{matrix}$

[0043] As a solution, one obtains: $\begin{matrix}{{{Q_{KAT}(t)} = {{\left\lbrack {\frac{T_{KAT}(0)}{c} - \left( {\frac{q_{zu}}{rc} + \frac{T_{U}}{c}} \right)} \right\rbrack \cdot {\exp \left( {- {rct}} \right)}} + \left( {\frac{q_{zu}}{rc} + \frac{T_{U}}{c}} \right)}}{and}} & \text{(A7)} \\{{T_{KAT}(t)} = {{\left\lbrack {{T_{KAT}(0)} - \left( {\frac{q_{zu}}{r} + T_{U}} \right)} \right\rbrack \cdot {\exp \left( {- {rct}} \right)}} + \left( {\frac{q_{zu}}{r} + T_{U}} \right)}} & \text{(A8)}\end{matrix}$

[0044] wherein: T_(KAT)(0) is the temperature in the E-catalyticconverter at the start of the viewing time frame, that is, at t=0.

[0045] The block circuit diagram of FIG. 2 shows the described thermalE-catalytic converter model.

[0046] In FIG. 3, an exhaust-gas after-treatment system 10 of aninternal combustion engine in accordance with the invention is shownhaving an exhaust-gas system 12 for conducting the exhaust gases A. AnE-catalytic converter 14 is mounted in the exhaust-gas system 12. TheE-catalytic converter 14 is driven by a control apparatus 16 forcarrying out the method of the invention.

[0047] It is understood that the foregoing description is that of thepreferred embodiments of the invention and that various changes andmodifications may be made thereto without departing from the spirit andscope of the invention as defined in the appended claims.

What is claimed is:
 1. A method for controlling an electrically heatablecatalytic converter for after treating exhaust gases of an internalcombustion engine, the method comprising the steps of: deactivating ornonactivating said catalytic converter during the start of said enginewhen the temperature (T_(KAT)(t_(abst))) of said catalytic converterexceeds a critical temperature T_(KRIT); and, utilizing a temperaturemodel to estimate said temperature (T_(KAT)(t_(abst))) of said catalyticconverter.
 2. The method claim 1, wherein said temperature modeldetermines said temperature (T_(KAT)(t_(abst))) in dependence upon theduration of switchoff when starting said engine.
 3. The method of claim2, wherein said temperature model determines said temperature(T_(KAT)(t_(abst))) when starting said engine in accordance with thefollowing relationship: T _(KAT)(t _(abst))=[T _(KAT)(o)−T_(U)]·exp(−r·c·t _(abst))+T _(U) wherein: T_(KAT)(t_(abst)) is thetemperature of said catalytic converter after a switchoff duration(t_(abst)) computed from a switchoff time point of said engine (t=0);T_(KAT)(0) is the temperature of the catalytic converter at the timepoint of the switchoff of said engine; T_(U) is the ambient temperature;r is a thermal transition resistance; and, c is a thermalproportionality constant.
 4. The method of claim 3, wherein saidswitchoff duration (t_(abst)) is read in via a combination instrumentand/or is determined from a model of the cool-down characteristic ofsaid engine.
 5. The method of claim 4, wherein said ambient temperature(T_(U)) is determined from the intake air temperature of said engine. 6.The method of claim 4, wherein a constant substitute value is used forsaid ambient temperature (T_(U)).
 7. The method of claim 3, wherein saidtemperature of said catalytic converter is derived from an exhaust-gastemperature model at the time point of switching off said engine.
 8. Themethod of claim 3, wherein a constant substitute value is used for saidtemperature of said catalytic converter at the time point of switchingoff said engine.
 9. A method for controlling an electrically heatablecatalytic converter for after treating exhaust gases of an internalcombustion engine, the method comprising the steps of: deactivating saidcatalytic converter after a required heating duration (T_(ekat));utilizing a temperature model to estimate said temperature(T_(KAT)(t_(abst))) of said catalytic converter; and, determining arequired heating characteristic and/or said required heating duration(T_(ekat)) from said temperature (T_(KAT)(t_(abst))) during start of theengine and from the ambient temperature (T_(U)).
 10. The method of claim9, wherein said heating characteristic and/or said heating duration(t_(ekat)) is derived from the following relationship:${T_{KAT}(t)} = {{\left\lbrack {{T_{KAT}(0)} - \left( {\frac{q_{zu}}{r} + T_{U}} \right)} \right\rbrack \cdot {\exp \left( {- {rct}} \right)}} + \left( {\frac{q_{zu}}{r} + T_{U}} \right)}$

wherein: T_(KATAB)(t_(ekat)) is the switchoff temperature of saidcatalytic converter after the required heating duration (t_(ekat));T_(KAT) (0) is the temperature of the catalytic converter at the timepoint of the switchoff of said engine; q_(zu) is the entered heat in thecatalytic converter because of the electric heating power thereof; T_(U)is the ambient temperature; r is a thermal transition resistance; and, cis a thermal proportionality constant.
 12. The method of claim 11,wherein the entered heat (q_(zu)) is determined by measuring the heatingvoltage and/or the heating current of the catalytic converter.
 13. Themethod of claim 12, wherein a constant substitute value is used for saidentered heat (q_(zu)).
 14. The method of claim 10, wherein said requiredheating duration (t_(ekat)) up to reaching said required switchofftemperature (T_(KATAB)) is determined by the following relationship:$t_{ekat} = {\frac{1}{r \cdot c} \cdot {{\ln \left( \frac{{T_{KAT}(0)} - \frac{q_{zu}}{r} - T_{U}}{T_{KATAB} - \frac{q_{zu}}{r} - T_{U}} \right)}.}}$


15. The method of claim 10, wherein the required heating duration(t_(EKAT)) up to reaching the needed switchoff temperature (T_(KATAB))is determined from continued computation of the catalytic convertertemperature (T_(KAT)) with the currently elapsed heating duration (t)and the subsequent comparison of the result to the switchoff temperature(T_(KATAB)).
 16. The method of claim 15, wherein the catalytic converteris deactivated or not activated when a diagnostic method diagnoses afault.
 17. The method of claim 16, wherein said fault is in the feedlines to said catalytic converter.
 18. The method of claim 16, whereinsaid catalytic converter is deactivated or not activated when there is adanger that the catalytic converter can freeze up at low ambienttemperature.
 19. The method of claim 16, wherein said catalyticconverter is deactivated or not activated when said engine stalls duringthe heating phase of said catalytic converter.
 20. An exhaust-gas aftertreatment arrangement of an internal combustion engine having anexhaust-gas system, the arrangement comprising: an E-catalytic convertermounted in said exhaust-gas system; and, a control apparatus forcarrying out the method including the steps of: deactivating ornonactivating said catalytic converter when starting said engine whenthe temperature (T_(KAT)(t_(abst))) of said catalytic converter exceedsa critical temperature T_(KRIT); and, utilizing a temperature model toestimate said temperature (T_(KAT)(t_(abst))) of said catalyticconverter.